Field of the Invention
The present invention concerns a method to control the acquisition of a diagnostic image data set of at least one part of a contrast agent-filled target area of a patient with a magnetic resonance device according to an acquisition protocol, as well as a magnetic resonance apparatus designed to perform such a method.
Description of the Prior Art
Contrast agent is often used within the scope of magnetic resonance imaging, in particular when image exposures should be produced within the context of dynamic processes, and for perfusion data acquisition in the vascular system of a patient. One important sign of quality of the image data sets that are acquired in this manner is that a good contrast agent filling is present. This means that the acquisition of the image data set must be implemented so that the acquisition region is optimally filled with contrast agent during the entire acquisition time. Magnetic resonance imaging thereby differs from other imaging modalities (for example computed tomography) because it is often a goal that data to be entered into the center of k-space are to be acquired at the point in time of the greatest contrast agent concentration in the acquisition region, since in this way the contrast-to-noise ratio (CNR) can be maximized. In other words: it is thus the goal to be able to cause the peak of the contrast agent concentration in the arterial or venous phase (depending on the acquisition goal) to occur simultaneously with the acquisition of the k-space center data.
In order to achieve this, it is known to use the test bolus technique within the scope of magnetic resonance angiography. In this technique, first bolus of the contrast agent (the test bolus) is initially injected that has a smaller amount of contrast agent than the main bolus to be administered for the diagnostic image acquisition, while first magnetic resonance images are acquired using a suitable magnetic resonance sequence in a first acquisition area that ideally corresponds to the acquisition area of the diagnostic image data set that is to be acquired later. These first magnetic resonance images can then be evaluated in order to determine the time from the administration of the test bolus to the time of highest contrast agent concentration in the acquisition area of the image data set (designated as peak time in the following). It is thus ultimately determined how long the test bolus requires in order to arrive in the acquisition area of the diagnostic image data set, and thus when the ideal point in time exists for acquisition of the k-space center.
After the test bolus measurement, the actual clinical examination is conducted. After the administration of the main bolus of the contrast agent, the acquisition protocol for the image data set is started only after a wait period that results from the peak time determined in the test bolus measurement. For example, such a procedure is described in an article by Thomas F. Hany et al., “Optimization of Contrast Timing for Breath-Hold Three-dimensional MR Angiography”, JMRI 1997; 7:551-556.
However, a number of problems occur in a test bolus measurement. It is possible that, due to a human error, the contrast agent is administered too early, for example, by a start signal for the wait period being provided with a time offset toward the injection of the contrast agent of the main bolus. This problem exists because the synchronization between the injection of the contrast agent and the start of the wait period must occur manually on the part of the user because, although contrast agent injectors that can automatically trigger the start of a wait period are known in computed tomography systems, an automatic injection controller for magnetic resonance systems is not yet known because it is extremely complicated to produce magnetic resonance-compatible devices. If the contrast agent is administered too early—meaning before the beginning of the wait period—this has the result that the acquisition protocol (and thus the measurement of the image data set) begins too late, such that the image data set overall has a poorer quality, in particular because venous portions can already be present in the arterial imaging and the like. In the worst case, the examination must be shifted to a further day because the amount of contrast agent that can be administered to a patient within a given time duration is limited.
An additional problem is the possibility of a variation of the physiological situation of the patient. For example, if the patient is excited, adrenalin can be produced, which increases the circulation speed so that it may occur, due to such physiological causes, that the contrast agent arrives before the wait period has ended, such that a non-optimal image data set is acquired.
Other variants in order to start the acquisition protocol for the image data set optimally at the correct point in time are likewise known in the prior art. Naturally it is possible to manually detect the main bolus. Magnetic resonance images of a second acquisition area (which is often selected at a certain distance from the acquisition area of the diagnostic image data set) can be acquired that have a high temporal resolution. For example, such magnetic resonance images can show a blood vessel that will feed the main bolus to the actual acquisition area of interest. When the arrival of the contrast agent is detected in the magnetic resonance images, the start of the acquisition protocol is triggered manually.
A further variation of this procedure is the automatic tracking of the main bolus (bolus tracking, also known under the keyword “care bolus”). Because the full dose of the contrast agent was injected as the main bolus, the arrival of the contrast agent in the second acquisition area is hereby automatically detected by a post-processing and evaluation of the magnetic resonance image that starts immediately after acquisition of a magnetic resonance image of the second acquisition area. The acquisition protocol is started automatically after the automatic detection of the contrast agent. For example, such a method is described in an article by C. Geppert et al., “Automatic Bolus Detection in Breast MRI: a method to improve accuracy and reliability?”, Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), 3085.
Such procedures for automatic bolus detection (“care bolus”) also have disadvantages. A first intrinsic problem of this variant is the selection of the second acquisition area, which does not correspond to the acquisition area of the image data set (and naturally also does not correspond to the first acquisition area in a test bolus measurement). This is due to the fact that some time is required in order to execute the start of the actual imaging, (acquisition protocols often include speech commands), and the time until the k-space center is measured can necessarily also be in the range of seconds. The estimation of a suitable second acquisition area is extremely difficult, and as a consequence a further problem of automatic bolus detection results from the poor time resolution of this method because, for example, data of new magnetic resonance images are present only every second or every two seconds. This poor time resolution, and the fact that the time that the blood (and thus the contrast agent bolus) requires in order to arrive from the second acquisition area to the acquisition area of the image data set is very short, have the result that compromises are often necessary in the quality of the image data sets. In this context, it is also known to use extremely short speech commands.